A rubber stopper for a vial, which is covered with a plastic sheet having a chemical resistance on the surface contacted with liquid medicine in order to prevent migration of vulcanization-accelerator, impurities and the like contained in the rubber to the liquid medicine, has been hitherto known (Japanese Examined Patent Publication No. 9119/1979, Japanese Unexamined Patent Publication No. 47637/1982).
Also, there has been known a rubber stopper for a vial, of which top surface is sometimes covered with a plastic sheet having good lubricity in order to improve transportability of the rubber stopper in the process of its production (Japanese Unexamined Patent Publication No. 296756/1988).
As a plastic sheet used in such a covered rubber stopper, a sheet of a fluorocarbon resin having an excellent chemical resistance and a fine smoothness surface has been used (Japanese Examined Patent Publication No. 9119/1979).
However, since the rubber stopper which is covered with the sheet of a fluorocarbon resin lacks in adhesive property to the rubber due to the inactivity of the fluorocarbon resin, after treating the sheet surface with sodium naphthalate for dehalogenation, spattering for etching the surface, adhesive agent for anchor effect and the like, the rubber stopper is adhered to the fluorocarbon resin. Therefore, the process for treating the surface of the sheet is not only complicated but also involves such risk that chemicals used during the abovementioned treatments slightly remain in the rubber stopper and the chemicals are mixed with the liquid medicine in the vial.
In order to solve these problems, the present inventors have eagerly studied. As a result, they have accomplished the present invention.